Step #6:

Use the back half of the kid's car and some scrap material (such as lumber or plastic if it's available) to create a support frame for the front end of the carrier.

Secure the material to the kid's car (via bolts and screws) and build outward to produce a frame (bringing the front end to a point is suggested because it will allow for a single pivot wheel and it will provide a strong structure).

Add a Wheel/Wheels

Attach a pivot wheel at the front end of your newly created structure.